This invention relates to a strip transmission line whose electric length may be adjusted by a variable premagnetizing field affecting the ferrite material with which a strip-shaped inner conductor and/or an outer conductor surrounding the inner conductor is at least partially coated.
A strip transmission line of the above-outlined type is disclosed, for example, in published European Application 429 791. Since the electric length of the strip transmission line is variable, it may be used, for example, for the tuning of resonators. Since such a tuning device for a strip transmission line may be operated with very high power because of its low losses and superior cooling possibilities, it may be advantageously used in particle accelerators.
The known strip transmission line whose electric length may be varied is exposed to a premagnetizing field which has static and variable components. The static premagnetizing field generated by a permanent magnet is of such a magnitude that the ferrite in the strip transmission line is premagnetized to saturation. In the saturated state the ferrite is loss-free and upon further increase of the premagnetizing field the microwave properties of the ferrite may be influenced to such an extent that the electric length of the strip transmission line changes. The variable component of the premagnetizing field is conventionally generated by an electromagnet.
It is a disadvantage of the known arrangement that the variable premagnetizing field is generated externally of the strip transmission line, and the magnetic field has to penetrate the outer and the inner conductors of the symmetrical strip transmission line to ensure that the magnetic field has an effect on the ferrite. Since the outer and inner conductors are made of an electrically conducting material, eddy currents are generated which prevent a rapid penetration of the magnetic field into the ferrite material. The tuning speed of the strip transmission line is thus limited by the eddy currents. For example, the limit frequency is approximately 25 Hz for the tuning of a strip transmission line whose housing which serves as the outer conductor is of brass having a wall thickness of 2 mm. The limit frequency for the tuning may be increased to approximately 800 Hz if, for example, a high-grade steel is used as the housing material.
By reducing the wall thickness of the housing, the limit frequency could be further increased. Such a possibility, however, is limited by the required mechanical stability of the arrangement.
A reduction of the eddy currents could further increase the limit frequency. By providing slots in the housing the paths of the eddy currents may be interrupted. In such a case, however, it has to be taken into consideration that the slots extend parallel to the current paths of the high-frequency field of the strip transmission line so as not to interrupt the high frequency currents which would lead to an energy scattering.
It has been found that all the above-outlined measures used in a tunable strip transmission line structured according to the state of the art make possible a maximum tuning frequency of approximately 2 kHz.